In Japan, four welding processes have been generally adopted for producing continuous welded rail (CWR): flash welding (FW), gas pressure welding (GPW), enclosed arc welding (EAW), and thermit welding (TW). Thermit welding method is the most popular rail welding process in Japan. The reliability of the thermit welds is high, because the failure rate has only been about 0.004 % over the last 10 years in Japan. However, recently there have been many cases where thermit welds are judged to be defective due to surface defects. Surface defects are often observed when welding new and worn rails. Welding test results show that surface defects are caused by gas generated from the contact of luting sand with molten steel which enters gaps between the welding mold and rail surface.